When the receiving mail server returns a temporary SMTP response code (4xx), HES mail tracking will place the mail in the deferred queue for retry. Mails in the deferred queue are retried periodically until time out is reached.
The action "queued for delivery" means that the mail is already in HES outbound MTA but for some reason, has not yet been accepted by the receiving mail server due to a temporary error.
Below is a list of temporary SMTP error response codes and what they mean:
- 421 - <domain> Service not available, closing transmission channel
- 450 - Requested mail action not taken: mailbox unavailable
- 451 - Requested action aborted: local error in processing
- 452 - Requested action not taken: insufficient system storage
If the receiving mail server returns any of the above codes, mail tracking will show the action as "queued for delivery".
To resolve this issue, please contact your mail server administrator.
Other items to consider:
- Make sure that your mail server IP/FQDN in the Administration > Domain Management page of the HES console is set to the correct MTA.
- If you are restricting traffic from specific IP ranges, make sure to allow traffic from HES servers. Follow the procedure in the article: Accepting emails coming from HES servers only.